For many years there has existed a desire for pressure-sensitive adhesive-coated polyolefin products (e.g., polyethylene, polypropylene or polyallomer, which is an ethylene:propylene copolymer) in which the pressure-sensitive adhesive is firmly bonded to the polyolefin and remains so at elevated temperatures, e.g., 120.degree. C. Although it is comparatively easy to bond rubber-resin type pressure-sensitive adhesives to polyolefin substrates, the adhesive bond tends to weaken and the product to be subject to failure at temperatures of about 65.degree. C. or higher. Although acrylic-type pressure-sensitive adhesives have superior heat resistance, they do not normally bond effectively to polyolefin substrates; indeed polyolefin films are sometimes used as release liners for these adhesives.
Untreated polyethylene and other polyolefins are poor substrates for acrylic adhesives, and a large amount of effort has gone into the search for a technique which will easily and reliably increase adhesion without significant changes in bulk properties. The most extensively studied techniques for enhancing adhesive bonding are treatment with helium gas plasma, oxygen gas plasma, or chromic acid. These and other surface modification procedures suffer from a common shortcoming in the poor durability of the treatment. Light rubbing of the surface causes a decrease in the effect, the altered surface being easily abraded. A more durable treatment would be the application to the polyolefin surface of a thin coating of material which is itself a good substrate for the adhesive. Then, however, the weak link becomes the interface between the polyolefin surface and the coating.
A particular application for polyolefin-backed pressure-sensitive adhesive coated products in which higher temperature resistance is desired is shown in Schams et al U.S. Pat. No. 4,216,257 and Kalleberg U.S. Pat. No. 4,290,174, which describe fasteners comprising two articles adapted for releasable engagement. At least one of the articles comprises a backing with a non-fibrous polyolefin surface bonding layer in which are embedded a plurality of U-shaped flexible resilient monofilaments of longitudinally oriented polymeric material. Each of the monofilaments includes two stem portions projecting from the surface bonding layer and terminating in enlarged heads having outer cam surfaces adapted to engage the other article. The manufacture of this product is described in Kalleberg U.S. Pat. No. 4,290,832; the disclosures of these three patents are incorporated herein by reference.
Fasteners of the type described in the aforementioned patents are particularly useful in attaching the covers to automobile seats. Automobile interiors, however, are often subjected to extremely high temperatures (e.g., 100.degree.-120.degree. C. or even higher), especially when a closed automobile remains in the sun for extended periods of time. The rubber-resin type normally tacky and pressure-sensitive adhesives referred to in the aforementioned Schams et al and Kalleberg patents are incapable of functioning effectively in such environments. It has been recognized that acrylic-type normally tacky and pressure-sensitive adhesives, such as disclosed in Ulrich U.S. Pat. No. Re. 24,906, are capable of enduring higher temperatures, and acrylic-type adhesives prepared in accordance with the teaching of Martens et al U.S. Pat. No. 4,181,752 are capable of maintaining their strength at perhaps even higher levels. Additional strength can be imparted to acrylic-type normally tacky and pressure-sensitive adhesives by crosslinking, e.g., as shown in the following U.S. Pat. Nos.: Stow 2,925,174, Ulrich 2,973,286, and Hendricks 2,956,904. In some instances, it has been found useful to employ highly conformable acrylic-type pressure-sensitive adhesives that contain uniformly distributed small glass microbubbles, as disclosed in Levens U.S. Pat. No. 4,223,067. Again, in the interest of completeness, the disclosures of these patents are likewise incorporated by reference.
Despite the recognized desirability of using acrylic-type normally tacky and pressure-sensitive adhesive compositions in the structures described above, there did not, prior to the present invention, exist any recognized way of securing adequate adhesion to polyolefin substrates.